Apparatus and method for cardiac ablation

ABSTRACT

A system and method for cardiac mapping and ablation include a multi-electrode catheter introduced percutaneously into a subject&#39;s heart and deployable adjacent to various endocardial sites. The electrodes are connectable to a mapping unit, an ablation power unit a pacing unit, all of which are under computer control. Intracardiac electrogram signals emanated from a tachycardia site of origin are detectable by the electrodes. Their arrival times are processed to generate various visual maps to provide real-time guidance for steering the catheter to the tachycardia site of origin. In another aspect, the system also include a physical imaging system which is capable of providing different imaged physical views of the catheter and the heart. These physical views are incorporated into the various visual maps to provide a more physical representation. Once the electrodes are on top of the tachycardia site of origin, electrical energy is supplied by the ablation power unit to effect ablation.

This is a continuation-in-part of application Ser. No. 08/029,771, filedMar. 11, 1993.

BACKGROUND OF THE INVENTION

This invention relates to medical devices and, in particular, a systemand technique of employing multi-electrode catheters for cardiac mappingand ablation.

Cardiac dysrhythmias are commonly known as irregular heart beats orracing heart. Two such heart rhythm irregularities are theWolff-Parkinson-White syndrome and atrioventricular (AV) nodal reentranttachycardia. These conditions are caused by an extraneous strand ofconducting fibers in the heart that provides an abnormal short-circuitpathway for electric impulses normally conducting in the heart. Forexample, in one type of Wolff-Parkinson-White syndrome the accessorypathway causes the electric impulses that normally travel from the upperto the lower chamber of the heart to be fed back to the upper chamber.Another common type of cardiac dysrhythmias is ventricular tachycardia(VT), which is a complication of a heart attack or reduction of bloodsupply to an area of heart muscle, and is a life threatening arrhythmia.All these types of dysrhythmias can usually be traced to one or morepathological “sites of origin” or tachycardia foci in the heart.

In the treatment of cardiac dysrhythmias, non-surgical procedures suchas management with drugs are favored. However, some dysrhythmias of theheart are not treatable with drugs. These patients are then treated witheither surgical resection of the site of origin or by Automaticimplantable cardiovertor defibrillator (AICD). Both procedures haveincreased morbidity and mortality and are extremely expensive. Even AICDneeds major surgical intervention. In addition, some patients ofadvanced age or illness cannot tolerate invasive surgery to excisetachycardia focus which causes dysrhythmias.

Techniques have been developed to locate sites of tachycardia and todisable their short-circuit function. The site of origin of tachycardiais determined by analysis of surface electrocardiogram or intracardiacelectrogram signals during states of arrhythmias which may occurspontaneously or be induced by programmed pacing. Once the site oforigin or focus is located, the cardiac tissues around the site areeither ablated surgically or with electrical energy so as to interruptabnormal conduction.

For cardiac mapping, several methods of gathering and analyzing surfaceelectrocardiogram or intracardiac electrogram signals are commonly used.

Surface electrocardiogram is one tool in which the electrocardiogramsare gathered from as many as twelve surface electrodes attached tovarious external body parts of a subject. The ensemble ofelectrocardiograms usually has a definite signature which may be matchedto that generally established to associate with a site of origin in agiven location of the heart. In this way, it is possible to determinethe gross location of a tachycardia site in the heart.

Intracardiac electrogram allows a tachycardia site of focus to belocated more accurately. It is obtained by detecting electrical signalswithin the heart by means of electrodes attached directly thereto.

Gallagher et al., “Techniques of Intraoperative ELectrophysiologicMapping”, The American Journal of Cardiology, volume 49, January 1982,pp. 221-240, disclose and review several methods of intraoperativemapping in which the heart is exposed by surgery and electrodes areattached directly to it. In one technique, the electrodes at one end ofa roving catheter are placed on a series of epicardial or endocardialsites to obtain electrograms for mapping earliest site of activationwith reference to surface electrocardiograms. For endocardial mapping, acardiotomy may also be necessary to open the heart to gain access to theendocardium.

Gallagher et al., supra, also disclose a technique for simultaneous,global mapping of the external surface of the heart (epicardialmapping). A lattice of about 100 electrodes in the form of a sock isworn on the heart, thereby enabling multiple sites to be recordedsimultaneously. This technique is particular useful for those caseswhere the ventricular tachycardia induced is unstable or polymorphic.

Global mapping by means of large array of electrodes has been furtherdisclosed in the following two journal articles: Louise Harris, M. D.,et al., “Activation Sequence of Ventricular Tachycardia: Endocardial andEpicardial Mapping Studies in the Human Ventricle,” Journal of AmericanCollege of Cardiology (JACC), Vol. 10, November 1987, pp. 1040-1047;Eugene Downar, et al., “Intraoperative Electrical Ablation ofVentricular Arrhythmias: A “Closed Heart” Procedure,” JACC, Vol. 10, No.5, November 1987, pp. 1048-1056. For mapping the interior surface of theheart (endocardial mapping), a lattice of about 100 electrodes in theform of a inflatable balloon is placed inside the heart after cutting itopen. Under some situations, a “closed heart” variation may be possiblewithout the need for both a ventriculotomy and ventricular resection.For example, with the subject on cardiopulmonary bypass, a deflatedballoon electrode array is introduced into the left ventricular cavityacross the mitral valve. Once inside the ventricle, the balloon isinflated to have the electrodes thereon contacting the endocardium.

While the sock or balloon electrode arrays allow global mapping byacquiring electrogram signals over a wider area of the heartsimultaneously, they can only be installed after open-chest surgery.

Catheter endocardial mapping is a technique for mapping the electricalsignals inside the heart without the need for open-chest or open-heartsurgery. It is a technique that typically involves percutaneouslyintroducing an electrode catheter into the patient. The electrodecatheter is passed through a blood vessel, like femoral vein or aortaand thence into an endocardial site such as the atrium or ventricle ofthe heart. A tachycardia is induced and a continuous, simultaneousrecording made with a multichannel recorder while the electrode catheteris moved to different endocardial positions. When a tachycardia focus islocated as indicated in intracardiac electrogram recordings, it ismarked by means of a fluoroscope image. Catheter endocardial mapping aredisclosed in the following papers:

M. E. Josephson and C. D. Gottlieb, et al., “Ventricular TachycardiasAssociated with Coronary Artery Disease,” Chapter 63, pp. 571-580,CARDIAC ELECTROPHYSIOLOGY—from cell to bedside, D. P zipes et al,Editors, W. B. Saunders, Philadephia, 1990.

M. E. Josephson et al., “Role of Catheter Mapping in the PreoperativeEvaluation of Ventricular Tachycardia,” The American Journal ofCardiology, Vol. 49, January 1982, pp. 207-220. Linear multipolarelectrode cathetersiare used in preoperative endocardial mapping.

F. Morady et al., “Catheter Ablation of Ventricular Tachycardia WithIntracardiac Shocks: Results in 33 Patients,” CIRCULATION, Vol. 75, No.5, May 1987, pp. 1037-1049.

Kadish et al., “Vector Mapping of Myocardial Activation,” CIRCULATION,Vol. 74, No. 3, September 1986, pp. 603-615.

U.S. Pat. No. 4,940,064 to Desai discloses an orthogonal electrodecatheter array (OECA). Desai et al., “Orthogonal Electrode CatheterArray for Mapping of Endocardiac Focal Site of Ventricular Activation,”PACE, Vol. 14, April 1991, pp. 557-574. This journal article disclosesthe use of an orthogonal electrode catheter array for locating problemsites in a heart.

Upon locating a tachycardia focus, ablation of cardiac arrhythmias istypically performed by means of a standard electrode catheter.Electrical energy in the form of direct current or radiofrequency isused to create a lesion in the endocardiac tissues adjacent (i.e.underneath) the standard electrode catheter. By creating one or morelesions, the tachycardia focus may be turned into a region of necrotictissue, thereby disabling any malfunctions.

Existing catheter mapping techniques typically rely on analysis ofrecorded electrograms. Locating the site of origin and tracking thewhereabouts of the catheter are at best tricky and time-consuming, andoften proved unsuccessful.

Thus, it is desirable, to have a catheter mapping and ablation systemwith precision and speed and able to provide comprehensive guidance on areal-time basis.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to treatventricular tachycardia and other cardiac dysrhythmias by improvedcatheter mapping and ablations.

It is an object of the present invention to provide a system which iscapable of rapid and accurate cardiac mapping.

It is another object of the present invention to provide a system whichis capable of efficiently and accurately locating and ablating a site oforigin of tachycardia.

It is another object of the present invention to provide accurateguidance for efficiently and accurately ablating an endocardial site byfilling it with successive catheter ablations of a smaller area.

It is yet another object of the present invention to provide real-timevisual maps indicating the relative positions of the electrodes, thetachycardia site of origin and the heart.

These and additional objects are accomplished by a system including amulti-electrode catheter selectively connectable to a mapping unit, anablation unit and a pacing unit. The system also includes a computer forcontrolling the various functional components. In one embodiment thesystem additionally includes a physical imaging unit which is capable ofproviding different views of a physical image of the multi-electrodecatheter percutaneously introduced into the heart of a subject.

Electrogram signals emanated from a tachycardia site of origin in theendocardium are detectable by the electrode array. Their arrival timesare processed to generate various visual maps to provide real-timeguidance for steering the catheter to the tachycardia site of origin.

In one embodiment, the visual map includes a footprint of the electrodearray on an endocardial site. The arrival time registered at eachelectrode is displayed in association therewith. A medical practitionercan therefore steer the catheter in the direction of earlier and earlierarrival time until the tachycardia site of origin is located.

In another embodiment, the visual map also includes isochrones which arecontours of equal arrival time. These isochrones are constructed bylinear interpolation of arrival times registered at the electrode arrayand cover the area spanned by the electrode array. When the electrodearray is far from the tachycardia site of origin, the isochrones arecharacterized by parallel contours. When the electrode array is close toor on top of the tachycardia site of origin, the isochrones arecharacterized by elliptical contours encircling the tachycardia site oforigin. Therefore, the isochrones provide additional visual aid andconfirmation for steering the catheter to the tachycardia site oforigin.

In another preferred embodiment, the visual map also includes anestimated location of the tachycardia site of origin relative to theelectrode array. This provides direct visual guidance for rapidlysteering the catheter to the tachycardia site of origin. The tachycardiasite of origin lies in the weighed direction of electrodes with theearliest arrival times. The distance is computed from the velocity andtime of flight between the site of origin and a central electrode. Thevelocity is estimated from a local velocity computed from theinter-electrode spacings and arrival time differentials.

According to another aspect of the invention, the system also include aphysical imaging system which is capable of providing different imagedphysical views of the catheter and the heart. These physical views areincorporated into the various visual maps to provide a more physicalrepresentation.

In one embodiment, two visual maps display two views (e.g., x,y axes) ofa physical image of the electrode array in the heart with a relativeposition for the tachycardia site of origin.

In another embodiment, a visual map display a three-dimensionalperspective view of the electrode array in the heart with a relativeposition for the tachycardia site of origin.

In yet another embodiment, the visual map also marks previous sites ortracks visited by the electrode array.

With the aid of the visual maps, the electrode array can locate thetachycardia site of origin rapidly and accurately. The system thendirects electrical energy from the ablation power unit to the electrodearray to effect ablation.

Additional objects, features and advantages of the present inventionwill be understood from the following description of the preferredembodiments, which description should be taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a multi-electrode cathetermapping and ablation system of the invention;

FIG. 2A illustrates the proximal end of the orthogonal electrodecatheter array (OECA) in its fully retracted position or mode;

FIG. 2B illustrates the OECA in its fanned-out mode;

FIG. 2C shows the footprints of the five-electrode OECA electrodes;

FIG. 3A illustrates the five electrodes of the OCEA positioned on a pairof orthogonal axes, each passing through a pair of peripheral electrodesand the central electrode;

FIG. 3B shows an example measurement of the OECA from one endocardialsite;

FIG. 3C illustrates the linear interpolation scheme applied to QuadrantI of the example shown in FIG. 3B;

FIG. 3D shows the construction of a complete local isochronal map forthe entire area cover by the OECA as shown in FIG. 3D;

FIG. 4 shows example traces of surface EKG and intracardiac electrogram;

FIG. 5 illustrates schematically the ventricle or other heart chamberdivided arbitrarily into four segments, and the isochrone maps obtainedfrom various locations;

FIG. 6A illustrates by an example the construction of the displacementvector of the electrode array to the estimated site of origin;

FIG. 6B is a display on the video monitor showing the relative positionsof the electrode array and the estimated site of origin, according to apreferred embodiment of the invention;

FIG. 6C illustrates a display according to another embodiment whichincludes the electrode array with the arrival times and the localisochrone map;

FIG. 7A is a synthesized display on the video monitor of a digitizedpicture of the heart and the electrode array therein taken along a firstaxis by the physical imaging system, and also showing the relativeposition of the estimated site of origin, according to another preferredembodiment of the invention;

FIG. 7B is a display on the video monitor showing a similar picture asin FIG. 7A but taken along a second axis by the physical imaging system;

FIG. 8 is a display on the video monitor showing the pictures of FIGS.7A and 7B simultaneously, according to another preferred embodiment ofthe invention;

FIG. 9 is a display on the video monitor showing a relative position ofthe estimated site of origin against a perspective picture of the heartand the electrode array which is synthesized from pictures recorded fromalong several axes by the physical imaging system, according to anotherpreferred embodiment of the invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic block diagram of a multi-electrode cathetermapping and ablation system 10 according to a preferred embodiment ofthe present invention.

The system 10 essentially comprises of three functioning units, namely amapping unit 20, an ablation unit 30 and a pacing unit 40. A computer 50controls the operation of each of the units and their cooperations via acontrol interface 52. The computer receives operator inputs from aninput device 54 such as a keyboard, a mouse and a control panel. Theoutput of the computer may be displayed on a video monitor 56 or otheroutput devices (not shown).

In the preferred embodiment the system 10 also includes a physicalimaging system 60. The physical imaging system 60 is preferably a 2-axisfluoroscope or an ultrasonic imaging system. The physical imaging system60 is controllable by the computer 50 via the control interface 52. Inone implementation, the computer triggers the physical imaging system totake “snap-shot” pictures of the heart 100 of a patient (body notshown). The picture image is detected by a detector 62 along each axisof imaging. It usually includes a silhouette of the heart as well asinserted catheters and electrodes, and is displayed by a physicalimaging monitor 64. Two monitors may be used to display the two imagesobtained along each of the dual axes. Alternatively, the two images maybe displayed side-by-side on the same monitor. A digitized image data isalso fed into the computer 50 for processing and integrating intocomputer graphics to be displayed on the video monitor 56.

A multi-electrode catheter 70 is selectively routed to each of the threefunctioning units 20, 30 and 40 via a catheter lead connector 72 to amultiplexor 80. Auxiliary catheters 90 or electrodes 92 are alsoconnectable to the multiplexor 80 via one or more additional connectorssuch as 94, 96.

During cardial procedures, the multi-electrode catheter 70 is typicallyintroduced percutaneously into the heart 100. The catheter is passedthrough a blood vessel (not shown), like femoral vein or aorta andthence into an endocardial site such as the atrium or ventricle of theheart. Similarly, auxiliary catheters 90 may also be introduced into theheart and/or additional surface electrodes 92 attached to the skin ofthe patient.

When the system 10 is operating in a mapping mode, the multi-electrodecatheter 70 as well as optional auxiliary catheters 90 function asdetectors of intra-electrocardiac signals. The surface electrodes 92serve as detectors of surface electrocardiogram signals. The analogsignals obtained from these multi-electrode catheters and surfaceelectrodes are routed by the multiplexor 80 to a multi-channel amplifier22. The amplified signals are displayable by an electrocardiogram (EKG)monitor 24. The analog signals are also digitized via an A/D interface26 and input into the computer 50 for data processing and graphicaldisplay. Further details of the data acquisition, analysis, and displayrelating to intracardiac mapping will be disclosed later.

When the system 10 is operating in an ablation mode, the multi-electrodecatheter 70 is energized by the ablation unit 30 under the control ofthe computer 50. An operator issues a command through the input device54 to the computer 50. The computer controls the ablation unit 30through the control interface 52. This initiates a programmed series ofelectrical energy pulses to the endocardium via the catheter 70. Apreferred implementation of the ablation method and device is disclosedin U.S. Pat. No. 5,383,917 by Desai, et al., the entire disclosurethereof is incorporated herein by reference.

When the system 10 is operating in a pacing mode, the multi-electrodecatheter 70 is energized by the pacing unit 40 under the control of thecomputer 50. An operator issues a command through the input device 54whereby the computer 50 controls through the control interface 52 andmultiplexor 80 and initiates a programmed series of electricalsimulating pulses to the endocardium via the catheter 70 or one of theauxiliary catheters 90. A preferred implementation of the pacing mode isdisclosed in M. E. Josephson et al., “VENTRICULAR ENDOCARDIAL PACING II.The Role of Pace Mapping to Localize Origin of Ventricular Tachycardia,”The American Journal of Cardiology, Vol. 50, November 1982, relevantportion of the disclosure thereof is incorporated herein by reference.

In an alternative embodiment, the ablation unit 30 is not controlled bythe computer 50 and is operated manually directly under operatorcontrol. Similarly, the pacing unit 40 may also be operated manuallydirectly under operator control. The connections of the variouscomponents of the system 10 to the catheter 70, the auxiliary catheters90 or surface electrodes 92 may also be switched manually via themultiplexor 80.

Mapping

An important advantage of the present invention is the capability ofallowing a medical practitioner to use a roving catheter to locate thesite of origin of tachycardia in the endocardium quickly and accurately,without the need for open-chest and open-heart surgery. This isaccomplished by the use of the multi-electrode catheter 70 incombination with real-time data-processing and interactive display bythe system 10.

Essentially, the multi-electrode catheter 70 must be able to deploy atleast a two-dimensional array of electrodes against a site of theendocardium to be mapped. The intracardiac signals detected by each ofthe electrodes provide data sampling of the electrical activity in thelocal site spanned by the array of electrodes. This data is processed bythe computer to produce a real-time display including arrival times ofintracardiac signals at each electrode, and a local isochrone map of thesampled site. By plotting contours of equal arrival time of theintracardiac signals, the local isochrone map is an expedient way ofindicating how close and where the electrode array is from the site oforigin. Also, at each sampled site, the computer computes and displaysin real-time an estimated location of the site of origin relative to theelectrodes, so that a medical practitioner can interactively and quicklymove the electrodes towards the site of origin.

A suitable multi-electrode catheter for use in the present invention isa five-electrode orthogonal electrode catheter array (OECA) that hasbeen disclosed in U.S. Pat. No. 4,940,064 to Desai. Relevant portions ofsaid disclosure are incorporated herein by reference.

FIG. 2A illustrates the proximal end of the orthogonal electrodecatheter array (OECA) 70 in its fully retracted position or mode.Because the catheter material has a “set” or “memory” it will normallyreturn to this retracted position. The OECA comprises an eight-frenchfive-pole electrode catheter 70. It has a central stylet 102 with fourperipheral or circumferential electrodes 112, 113, 114 and 115. A fifthelectrode 111 is located centrally at the tip of the stylet 102. Allfive electrodes are hemispherical and have individual leads 116connected thereto. Each peripheral electrode is 2 mm in diameter whilethe central electrode is 2.7 mm in diameter. Slits 120 are cutlongitudinally near where the electrodes are located.

FIG. 2B illustrates the OECA in its fanned-out mode. When the proximalend (not shown) of the catheter is pulled, the stylet's slits 120 allowfour side arms 122 to open from the stylet body in an orthogonalconfiguration. Each of the four arms 122 extend a peripheral electroderadially from the stylet so that the four peripheral electrodes forms across with the fifth electrode 111 at its center. The inter-electrodedistance from the central electrode to each peripheral electrode is 0.5cm, and the distance between peripheral electrodes is 0.7 cm. Thesurface area of the catheter tip in an open position is 0.8 cm².

FIG. 2C shows the footprints of the five-electrode OECA electrodes. Thefour peripheral electrodes 112, 113, 114 and 115 or (2)-(5) form a crossconfiguration. The fifth electrode 111 or (1) is located at the centerof the cross. The orthogonal array of electrodes therefore provides fivesampling points over the zone 130 in an endocardium site spanned by theelectrodes.

Isochrone Maps

Generally when a patient's heart is in a state of tachycardia, the siteof origin becomes the source of endocardial activation, emanating aseries of activation wavefronts therefrom. Electrodes such as thosedeployed by the catheter 70 in the endocardium and located closer to thesite of origin will detect these wavefronts earlier than those furtheraway. The surface electrodes 92 being the furthest away from the site oforigin will generally register latest wavefront arrival times.

When an endocardial site is being mapped by the OECA, a singlemeasurement of an activation wavefront will provide arrival times at thefive electrodes in real time. A local isochrone map for the sampled sitecan then be constructed from these arrival times, thereby showingcontours of equal arrival times. The isochrones are readily computed bythe computer using a linear interpolation scheme, as illustrated below.

FIG. 3A illustrates the five electrodes of the OCEA positioned on a pairof orthogonal axes. Each orthogonal axes passes through a pair ofperipheral electrodes and the central electrode, viz 112-111-114 (or(2)-(1)-(4)) and 113-111-115 (or (3)-(1)-(5)). To implement the linearinterpolation scheme, the zone spanned by the five electrodes is bestdivided into four triangular quadrants I to IV. Quadrant I is bounded byelectrodes (1), (2), and (3). Quadrant II is bounded by electrodes (1),(3), and (4). Quadrant III is bounded by electrodes (1), (4), and (5).Quadrant IV is bounded by electrodes (1), (5), and (2). The localisochrones are then computed for each quadrant separately using linearinterpolation along each side of the triangle.

FIG. 3B shows an example measurement of the OECA taken from oneendocardial site. The five electrodes [(1), (2), (3), (4), (5)] eachrespectively has arrival time of [t(1), t(2), t(3), t(4), t(5)]=[−16,−6, −8, −20, −14] msec.

FIG. 3C illustrates the linear interpolation scheme applied to QuadrantI of the example shown in FIG. 3B. Quadrant I is a triangle defined bythe electrodes [(1), (2), (3)], each respectively having arrival timesof [t(1), t(2), t(3)]=[−16, −6, −8] msec. Taking one millisecond steps,the side defined by electrodes (1) and (2) can be divided into ten equalsteps from t=−6 to −16 msec. Similarly, the side defined by electrodes(2) and (3) can be divided into two equal steps from t=−6 to −8 msec,and the side defined by electrodes (1) and (3) can be divided into eightequal steps from t=−8 to −16 msec. Thus, an isochrone for the arrivaltime of −10 milliseconds can easily be drawn by joining a line from the−10 msec coordinate along each side. In this instance, the −10 mseccoordinate is found only along the two sides defined by electrodes (1)and (2) and electrodes (1) and (3).

FIG. 3D shows the construction of a complete local isochronal map forthe entire area covered by the OECA. The complete local isochronal mapis obtained by applying the linear interpolation method to all quadrantsfor all desired arrival times.

The activation-wavefront arrival time at each electrode is measuredrelative to a reference time. The reference time is usually provided bythe earliest deflection in a surface electrocardiogram which ismonitored throughout the cardiac procedure.

FIG. 4 shows typical example traces of surface EKG and intracardiacelectrograms. The top three traces are three surface electrocardiogramsI, AVF and V1, representing three planes (right to left,superior-inferior, anterior-posterior). These are continuously monitoredand the earliest deflection on any of these electrocardiograms serves asa reference point of time. In this example, a perpendicular dotted line(reference time zero) is drawn from the earliest surface EKG whichhappens to be lead I. The next five traces are unipolar intracardiacelectrograms as detected by an orthogonal electrode array catheter. Itcan been seen that electrode number. 5, having the earliest arrival timeof −36 msec is closer to the site of origin than the others.

It has been determined that an arrival time of −40 to −45 msec indicatesthat the detecting electrode is located substantially at the site oforigin. In this case, the OECA yields a local isochrone mapcharacterized by elliptical contours centered on the central electrode.On the other hand, when the OECA is substantially far away from the siteof origin, its local isochrone map is characterized by parallelcontours. The characteristic arrival time and associated isochronalsignature are useful for locating the site of origin.

The intracardiac and surface EKGs are preferably digitized using asimple 8 or 16 channel signal digitizer. When the system 10 is in themapping mode, the intracardiac electrograms and surface EKGs obtainedfrom the multi-electrode catheter 70 and the surface electrodes 92 aredigitized by the A/D interface 26. The digitized waveforms are analyzedby the computer to find the activation wavefront arrival times in realtime.

The method of operation of the inventive system in mapping mode will nowbe described by way of an example as follows. The multi-electrodecatheter 70 is first used in mapping. The catheter is inserted throughthe leg artery (right femoral) and advanced to the aortic arch and thento the left ventricle utilizing fluoroscopic guidance as provided by thephysical imaging system 60.

FIG. 5 illustrates schematically the ventricle or other heart chamberdivided arbitrarily into four segments, right-upper (RUS) andright-lower (RLS), and left-upper (LUS) and left-lower (LLS) segments.In the example shown, a site of origin 200 is located in the (LLS)segment. The catheter 70 (OECA) is used to sample each of the segmentsin order to identify the segment containing the site of origin 200. TheOECA is first positioned in the right upper segment, and its orthogonalelectrode array is deployed to measure arrival times of wavefrontactivation from the site of origin. The system 10 is then instructed toinitiate tachycardia by means of programmed electrical stimulationprotocol from the pacing unit 40 to an electrode inserted into theendocardium. Once tachycardia is induced, the OECA picks up theintracardiac activation wavefront arrival times which are analyzed bythe computer and a local isochrone map is displayed on the video monitor56. In the example shown in FIG. 5, when the OECA is in the (RUS)segment, all electrodes register a rather late arrival time, whichindicates that the site of origin is not in the (RUS) segment.

Next, the catheter electrodes are retracted and the catheter moved tothe lower segment (RLS). In this way all four segments are mapped. Inthe example shown, the catheter is eventually repositioned in thesegment (e.g., LLS) that demonstrates earliest arrival times.

Once the segment containing the site of origin has been identified,further manipulations of the catheter in that segment are undertakenwith the interactive aid of the display on the video monitor 56. Thedisplay shows in real-time the local isochrone map, the electrode arrayand the estimated position of the site of origin relative thereto.

FIG. 6A illustrates by an example the construction of the displacementvector of the electrode array to the estimated site of origin 201. Shownin FIG. 6A are the five electrodes of the OECA which are identical tothe ones shown in FIG. 3A. The example shown has the five electrodes[(1), (2), (3), (4), (5)] each detecting an activation wavefront arrivaltime respectively of [t(1), t(2), t(3), t(4), t(5)]=[−36, −27, −32, −40,−31] msec. The orthogonal interelectrode spacing is R=5 mm in this case.As explained before, the goal is to locate the electrode array centrallyabout the actual site of origin. Since the site of origin is the sourceof the activation wavefronts an electrode located at the site willdetect the earliest possible arrival time (typically −40 to 44 msec withrespect to the first deflection of the surface EKG). The goal isachieved by having the central electrode (1) detecting the earliestpossible arrival time. Conversely, when the electrode array is displacedfrom the site of origin, those electrodes further away from the site oforigin will detect arrival times later (less negative) than those thatare closer (more negative). Thus, the electrode array must be movedalong the direction of more negative arrival time in order to close inon the site of origin.

According to one embodiment, the direction in which the displacementvector joining the center of the electrode array to the estimated siteof origin 201 is determined by linear interpolation of the respectivearrival times detected at the five electrode locations. This can beeasily performed by treating each arrival time as an “equivalent mass”located at each electrode and calculating the “center of mass” for theelectrode array. The position of the “center of mass” is then given by:$\begin{matrix}{\left\lbrack {R_{x},R_{y}} \right\rbrack = {\left\lbrack {\frac{\sum\limits_{i}\quad{{r_{x}(i)}{t_{x}(i)}}}{\sum\limits_{i}\quad{t_{x}(i)}},\frac{\sum\limits_{j}\quad{{r_{y}(j)}{t_{y}(j)}}}{\sum\limits_{j}\quad{t_{y}(j)}}} \right\rbrack.}} & (1)\end{matrix}$

The OECA conveniently defines a set of orthogonal axes with an (x,y)coordinate system, viz: the direction along electrodes (1)-(2) being they-axis and the direction along electrodes (1)-(3) being the x-axis. Theexample data yield the position of the “center of mass” relative to theelectrode (1): $\begin{matrix}\begin{matrix}{\left\lbrack {R_{x},R_{y}} \right\rbrack = \left\lbrack {\frac{{{- 32}*R} + {\left( {- 31} \right)*\left( {- R} \right)}}{{- 32} + \left( {- 31} \right)},\frac{{{- 27}*R} + {\left( {- 40} \right)*\left( {- R} \right)}}{{- 27} + \left( {- 40} \right)}} \right\rbrack} \\{= {\left\lbrack {0.016,{- 0.19}} \right\rbrack R}}\end{matrix} & (2)\end{matrix}$where R=orthogonal interelectrode spacing (e.g. =5 mm).

The estimated site of origin 201 then lies along a directionD{circumflex over ( )} defined by a line through the central electrode(1) and the “center of mass”, [R_(x),R_(y)].

According to one aspect of the invention, the distance, |D|, between thecentral electrode (1) and the site of origin is estimated by firstdetermining the local w|D|=v _(D) |t(f)−t(1)|  (3)avefront velocity, v_(D), along the direction D{circumflex over ( )}.Thus, where

-   -   t(f)=arrival time measured at the site of origin,    -   t(1)=arrival time measured at the central electrode (1).

In the case of the OECA, it is expediently accomplished by firstcomputing the wavefront velocities along the x- and y-axis. This isestimated by the speed the wavefront travel from one electrode toanother along the x- and y-axis: $\begin{matrix}{\left\lbrack {v_{x},v_{y}} \right\rbrack = \left\lbrack {\frac{R}{\Delta\quad t_{x}},\frac{R}{\Delta\quad t_{y}}} \right\rbrack} & (4)\end{matrix}$

where R=interelectrode spacing, and the appropriate Δt_(x), Δt_(y) aregiven by the table below corresponding to the quadrant containing thedirection D{circumflex over ( )}: QUADRANT Δt_(x) Δt_(y) (1)-(2)-(3)t(1)-t(3) t(1)-t(2) (1)-(3)-(4) t(1)-t(3) t(1)-t(4) (1)-(4)-(5)t(1)-t(5) t(1)-t(4) (1)-(2)-(5) t(1)-t(5) t(1)-t(2)

The local wavefront velocity v_(D) is estimated by adding the componentof v_(x) and v_(y) along the direction D{circumflex over ( )}, viz.:v _(D) =v _(x) cos θ+v _(y) sin θ  (5)where θ tan⁻¹(R_(x)/R_(y)) is the angle between D{circumflex over ( )}and the x-axis.

In the example given in FIG. 6A, the direction D{circumflex over ( )}lies within the quadrant (1)-(3)-(4). Then Equation (4) yields$\left\lbrack {v_{x},v_{y}} \right\rbrack = {\left\lbrack {\frac{1}{- 4},\frac{1}{4}} \right\rbrack{R/{ma}}}$and Equation (5) yieldsv _(D)=0.25 R(−cos θ+sin θ)/msec≈−0.25 R/msec.

If the site of origin is assumed to have a measured arrival time oft(f)=−44 msec, then from Equation (3) the central electrode is displacedfrom the estimated site of origin 201 by a distance:D=v _(D)(44−36)=2R or 10 mm.

FIG. 6B illustrates a computer graphical display on the video monitor 56(see FIG. 1) in the preferred embodiment. The display shows, in realtime and simultaneously, the electrode array with its local isochronemap and the relative position of the estimated site of origin 201. Thisgreatly facilitates a medical practitioner to quickly steer theelectrode catheter array to the site of origin. As the electrodecatheter array is moved towards the estimated site of origin 201, theisochrones should be more and more elliptical. When the centralelectrode 111 is on top of the estimated site of origin, the isochronesshould be ellipes wrapping around the central electrode 111. If this isnot the case, t(f) needs to be revised and preferably changed in stepsof 2 msec at a time, until the event when coincidence of the centralelectrode with the estimated site of origin is accompanied by ellipticalisochrones wrapping around the central electrode.

As described earlier, Equation (1) is a linear interpolation schemebased on representing the arrival time at each electrode with anequivalent mass; the earlier the arrival time, the more “massive” it is.In this way, the data collected by every electrode in the array is takeninto consideration. The equation as it stands is applicable if all thearrival time is negative, which is the case when the electrodes are nottoo far off-field from the site of origin. In general, to accommodatealso positive or mixed positive and negative values of arrival times, itis expedient to shift all the arrival time values to the same polaritywith the view of having the earliest arrival time represented by thelargest value. In one embodiment, the arrival times are translated bythe formulaet_(x)→T₀−t_(x)t_(y)→T₀−t_(y)where T₀ is a positive constant larger than any of the positive arrivaltime values. For example, T₀=50, and the calculation in Equation (2)yields [R_(x), R_(y)]≈[1, −13]R.

FIG. 6C illustrates a display according to another embodiment whichincludes the electrode array with the arrival times and the localisochrone map. In this embodiment, an arrow 313 indicates the estimateddirection in which the catheter array should move in order to approachthe site of origin. In general, as the catheter array is moved from siteto site, there will be a map such as that illustrated in FIG. 6 cassociated with each site, with the current display showing the readingfrom the current site.

A further feature is the ability to store maps from previous sites andto recall these “history” information as needed. In one embodiment,another arrow 311 associated with the previous site is also displayed onthe current map to provide a line of reference from the previous site.The previous arrow is displayed with a different attribute such as withbroken line or with a different color in order to distinguish over thepresent arrow. In this way, the operator maneuvering the catheter willbe able to tell whether the current catheter position is getting closerto the site of origin relative to the last one. In another embodiment,the previous map is display in a smaller window in one corner of thecurrent map.

In yet another embodiment, as the catheter is mapping from site to site,the operator is able to mark the sites interactively on a graphicalterminal. Typically, on the graphical terminal is displayed a schematicdiagram of the heart such as the one shown in FIG. 5, and by referenceto a flouoscopic image of the catheter in the heart, an operator canmark the equivalent site on the schematic diagram. Each marker on theschematic diagram is linked to its associated map or associatedinformation. Subsequently, the operator is able to point to any existingmarker and recall its associated map or information.

Physical Image Integration

The computer video display shown in FIG. 6B is constructed essentiallyfrom information obtained through data processing of wavefrontarrival-time data sampled by the electrode catheter array 70. Thedisplay is an arrival-time field that exists in a two-dimensional spaceon the surface of the endocardium. For the purpose of locating thecatheter at the site of origin, it provides adequate and cost-effectiveguidance.

According to another aspect of the invention, the information obtainedby the physical imaging system 60 (see FIG. 1) is also integrated withthe information obtained from the wavefront arrival-time data. The twotypes of information are synthesized by the computer 50, and aredisplayed on the video monitor 56 as a physical image of the heart 100and showing therein the relative positions of the electrode catheterarray 70 and the estimated site of origin 201. In this way a morephysical representation of the catheter and heart is possible.

FIG. 7A is a synthesized display on the video monitor of a digitizedpicture of the heart 100 and the electrode array 70 therein taken alonga first axis by the physical imaging system, and also showing therelative position of the estimated site of origin 201, according toanother preferred embodiment of the invention.

In one implementation, the physical imaging system 60 (also see FIG. 1)comprises two x-rays taken from two perpendicular directions. The videooutput of both x-ray machines is digitized, e.g., by using two separatevideo frame grabbers integrated into the x-y detectors 62. Since theelectrode array 70 such as the OECA (also see FIGS. 2 and 3) has anx-ray opaque dart (not shown) on one of the electrode arms, it isrelatively simple for the computer to properly identify each electrodeand associate the correct arrival time with each electrode. In this way,the positions of the five electrodes of the OECA can be tracked by thecomputer 50 in real time.

The estimated site of origin 201 can be located by the method describedearlier, except the coordinate system may be non-orthogonal, dependingon the orientation of the electrode array.

FIG. 7B is a display on the video monitor showing a similar picture asin FIG. 7A but taken along a second axis by the physical imaging system.

The views from the two axes may be displayed on two separated videomonitors or on one monitor.

FIG. 8 is a display on the video monitor showing the pictures of FIGS.7A and 7B simultaneously, according to another preferred embodiment ofthe invention.

According to another embodiment of the invention, the video display is aperspective rendering of a three-dimensional image of the heart and theelectrode array.

FIG. 9 is a synthesized display on the video monitor of a perspectivepicture of the heart 100 and the electrode array 70 together with theestimated site of origin 201, according to another preferred embodimentof the invention. The image of heart 100 and the electrode array 70 arerendered from a three-dimensional image database which is collected fromimaging along several axes by the physical imaging system. Each axisprovide a view of the heart and the electrode array. The procedure forlocating the estimated site of origin in each view is similar to thatdescribed before. The data gathered from the different views areprocessed by the computer to generate a three-dimensional perspectiveview. In one implementation, sites previously visited by the catheter 70are also displayed as a track 211 in the endocardium.

The present inventive system is advantageous in allowing a medicalpractitioner to graphically track in real time the relative positions ofthe electrode array with respect to the heart and the estimated site oforigin. Furthermore, it allows the possibility of accurate catheterpositioning and repositioning in the endocardium and the possibility oftracking the history of the catheter previous positions.

Global Mapping

In preoperative studies and diagnosis or in medical research, a globalmapping of the heart is valuable. A global isochronal map for the entireendocardium is assembled by the catheter scanning over the entireendocardium and the computer piecing together the local isochrone mapsat each scanned site. The display includes tracks traversed by thecatheter to provide guidance so that the endocardium can be mappedsystematically. This will not only allow the computer to produce anddisplay local isochronal maps in real time, but also separate isochronalmaps of a larger area up to the whole endocardium by storing the actualpositions of the electrodes for each measurement and the correspondingarrival times. As each additional measurement is taken, the (non-local)isochronal map could be updated to cover a larger area more accurately.This would allow the medical practitioner conducting a medical procedureto determine where to place the OECA next for measurement and to decidewhether or not accurate enough isochronal map for the entire endocardiumhas been produced. Once an accurate enough isochronal map of theactivation wavefront has been produced, a proper treatment procedurecould then be determined.

Multi-Phase Radio Frequency Ablation

A preferred implementation of the ablation method and device isdisclosed in copending and commonly assigned U.S. patent applicationSer. No. 07/762,035 filed Jul. 5, 1991 by Desai, et al., the entiredisclosure thereof is incorporated herein by reference.

After the site of origin is located by the electrode array, the system10 (FIG. 1) is switched to the ablation mode. Electrical energy istransmitted from the ablation power unit 30 through the multiplexor 80to the electrode array catheter 70. In the preferred embodiment, theablation power unit 30 is programmable and under the control of thecomputer 50, so that a predetermined amount of electrical energy isdelivered to ablate the endocardium.

In catheter ablation, the lesion formed is about the size of theenergized electrode or electrode array. Conventional catheter ablationtechniques have typically employed a catheter with a single electrode atits tip as one electrical pole. The other electrical pole is formed by abackplate in contact with a patient's external body part. Thesetechniques have been used to disable the tachycardia site of origin inmost cases. For example, it has been successfully used for theinterruption or modification of conduction across the atrioventricular(AV) junction in AV nodal reentrant tachycardia; or for the interruptionof accessory pathway in patients with tachycardia due toWolff-Parkinson-White Syndrome; and for ablation in some patients withventricular tachycardia (VT).

However, in ventricular tachycardia (VT), endocardial mapping with astandard electrode catheter can locate the exit site of ventriculartachycardia to within 4-8 cm² of the earliest site recorded by thecatheter. A standard electrode catheter typically has a maximumelectrode tip area of about 0.3 mm². Therefore, the lesion created bythe simple RF technique delivered through a standard electrode cathetermay not be large enough to ablate the ventricular tachycardia. Attemptsto increase the size of lesion by regulation of power and duration byincreasing the size of electrode or by regulating the temperature of tipelectrode have met with partial success.

In order to increase the size of the lesion, the orthogonal electrodecatheter array (OECA) with four peripheral electrodes and one centralelectrode provides a larger footprint. It typically produces a lesionsof 1 cm².

However, in the ablative treatment of ventricular tachycardia (VT),lesion size of the order of more than one cm² is probably required foreffective treatment. In this case, a large lesion is formed bysuccessive ablation of adjacent sites. For example, a larger lesion of 6cm² size can be created by six adjacent square-shaped lesions of 1 cm².They can be formed by successive placements of the five-electrode OECAusing RF energy. After each ablation, the electrode catheters is usuallywithdrawn to clean blood coagulum on the electrodes before the nextattempt. It is critical that the locations of the next spot to beablated as well as the reintroduced catheter must be known accuratelyand quickly for this procedure to be successful. This is accomplished byswitching the system 10 alternately between the mapping and ablationmode. In the mapping mode, the system is preferably programmed tosuperposition a grid about the displayed tachycardia site such as thatshown in FIG. 7, 8 or 9. The grid will enable accurate positioning ofthe electrode array.

While the embodiments of the various aspects of the present inventionthat have been described are the preferred implementation, those skilledin the art will understand that variation thereof may also be possible.The device and method described therein are applicable to ablation ofbiological tissues in general. Therefore, the invention is entitled toprotection within the full scope of the appended claims.

1. A cardiac mapping system for locating a tachycardia site of origin inan endocardium of a subject's heart, comprising: catheter means fordisposing a cluster of electrodes about the endocardium site-by-site,each electrode capable of detecting intracardiac electrogram signalsemanating from the tachycardia site of origin; means responsive to theintracardiac electrogram signals detected at each electrode forcomputing an arrival time of the intracardiac electrogram signalsthereat; means for interactively displaying a map derived from saidarrival times of intracardiac electrogram signals, said map includingthe cluster of electrodes and a display of arrival time associated witheach electrode, whereby those electrodes being closer to the tachycardiasite of origin than others will register earlier arrival times thanothers, and electrodes that are substantially coincident with thetachycardia site of origin will register an earliest possible arrivaltime, thereby said map providing guidance for moving said catheter inthe direction of those electrodes having earlier arrival times; andmeans for storing and displaying a map obtained from a previous site.